Nuclear power plant uses heat generated by nuclear fission of uranium, and an UO2 sintered pellet is generally used as nuclear fuel for nuclear power plant. The UO2 sintered pellet may be produced by sintering a green pellet. The UO2 sintered pellet produced by such an existing method has a density of about 96% TD (theoretical density) and a grain size of about 8-14 μm.
Recently, high burnup nuclear fuels have been developed, which are burnt for a long time in order to increase economic efficiency of nuclear fuel and reduce an amount of spent fuel. As the burnup of nuclear fuel increases, a generation amount of fission products such as Xe, Kr, Cs and I increases. The increased fission product will increase stress in a cladding tube, which may deteriorate the safety of nuclear fuel. Accordingly, in order to overcome those limitations, fission product must be released from the pellet as little as possible.
In addition, after Hukusima nuclear power plant accident, there are increasing demands for development of UO2 nuclear fuel pellets with an enhanced accident resistance to trap highly radioactive fission product as much as possible to prevent the release of fission products having a high level of radioactivity outside the environments.
The fission product is the material generated during the fission caused after fissile materials (a typical example of such fissile materials is U-235) absorb thermal neutron. When fission occurs in UO2 nuclear fuel, one uranium atom is split into two fission products. Fission products can be classified into four groups in terms of their volatility and chemical activity: volatile fission product including fission gases, semi-volatile fission product, fission product that are low volatile, and non-volatile fission product. Among the four groups, volatile fission product (I and Cs) and fission gases (Xe and Kr), are most important in terms of fuel degradation and radiological consequence, because they have very strong chemical activity and are also easily released outside the fuel pellet and environment.